1. Field of the Invention
The invention pertains to the field of optoelectronic devices. More particularly, the invention pertains to semiconductor light emitting devices.
2. Description of Related Art
There is a need in the light sources providing a highly directional emission of photons. Furthermore, in many cases there is a need that the emission amplified is perpendicular to the surface of the device, or has the strongest component of the k-vector in this direction. However, in most of the cases the gain medium emits photons in a broad range of angles. A good overview of the problem can be found in High Power Diode Lasers: Fundamentals, Technology, Applications; R. Diehl, ed., Springer, Berlin (2000).
To counteract this effect several approaches were introduced. In light-emitting diodes photonic crystal structures surrounding the light-emitting medium can be introduced. These structures can suppress light emission in certain directions in the angle space, while enhance the light emission in the other directions. This concept can be used also in lasers, including laser diodes. Some examples of this approach include U.S. Pat. No. 6,704,343, entitled “HIGH POWER SINGLE MODE VERTICAL CAVITY SURFACE EMITTING LASER”, issued Mar. 9, 2004, and U.S. Pat. No. 6,810,056, entitled “SINGLE MODE VERTICAL CAVITY SURFACE EMITTING LASER USING PHOTONIC CRYSTALS WITH A CENTRAL DEFECT”, issued Oct. 26, 2004.
In lasers the directionality can be also improved by choosing a proper resonator providing a feedback only for the appropriate direction. However, generally, the resonator may also provide feedback for many optical modes within a certain angle space resulting in multimode light emission. In edge-emitting lasers, the angle spread is defined by the appearance of several transverse modes due to the large lateral or vertical size of the light-emitting aperture. The spacing between the longitudinal modes is very small in edge emitting lasers and the wavelength range of the different transverse lobes overlap. However, in single longitudinal mode edge emitting lasers such as distributed feedback lasers, appearance of different transverse modes also results in the evolution of several emission lines in the wavelength space. In vertical cavity surface emitting lasers with a significantly large light-emitting aperture (typically beyond 1 micrometer in diameter), the spread of the emission in the angle and the wavelength space are interconnected and defined by the appearance of high order transverse modes with emission tilted with respect to the normal to the interfaces in the device for the same longitudinal mode. In single mode devices having a small aperture, only one optical mode may be supported and the lasing occurs then in a single mode. However, also in this case the spread of the emission in the angle space is broadened due to the light diffraction effect at the small output aperture. Thus, also in this case, the angle component of emission, which is directed close to the normal to the surface of the device, may contribute to only a small fraction of the emission within the total diffraction-defined emission angle. In optical amplifiers stimulated emission may occur in all the directions defined by the light waveguiding structure. Even when a single mode single wavelength emission is coupled to the amplifier, several modes may be excited in the waveguide causing an angle spread at the exit of the device. When the waveguide supports only a single transverse mode the angle spread will be significant due to the light diffraction effect at the output aperture. Thus, to get a highly directional emission one may need to preferably amplify the optical emission in the preferable direction, in the case under consideration in the direction perpendicular to the interfaces in the multilayer structure or in the direction, which is the close to this perpendicular direction.